A hall in the King’s palace at Memphis.
The high priest Ramfis informs Radamès, the captain of the guards, that the Ethiopians have rebelled and are threatening the Nile valley. The goddess Isis has just told him the name of the man who is to command the Egyptian army, and the high priest goes off to inform the King.

Left alone, Radamès cherishes the secret hope that he himself has been selected and revels in the thought of his glory and above all of Aida, the Ethiopian slave whom he passionately loves. The expression on his face does not escape the notice of Amneris, the King’s daughter, who secretly loves him and fears a rival. When Aida enters, Radamès’s agitation and that of the slave herself heighten Amneris’s suspicions.

Preceded by guards, ministers and priests, the King enters. A Messenger tells of the invasion by the Ethiopians who, led by their king Amonasro, are about to attack Thebes.

The King informs the assembled company, who invoke war, that Isis has chosen Radamès to command the army. Radamès exults. Amneris hands him a flag and everyone exhorts him to return as the winner. Aida alone is desperate. For she cannot hope to see her people defeated and Amonasro, her father, dragged in chains, or to lose her beloved Radamès.

Interior of the temple of Vulcan at Memphis.
Ramfis and the priests stand beneath the altar. From within is heard the song of the priestesses invoking the god Fthà. Radamès is ushered into the temple. While the priestesses dance, a veil is spread over the army chief's head and he is invested with the sacred arms. Ramfis entrusts to him the fate of Egypt.